1. Field of the Invention
The invention relates in general to a compensation method and related circuits for baseline wander of a transmission signal, and more particularly, to a method and related circuits to compensate baseline wander of a transmission signal by an accumulation result according to numbers of a plurality of pulses for different digital data.
2. Description of the Prior Art
With the development of an Internet communication system, people all over the world are capable of delivering lots of information to each other in high speed, and which improve the spread of knowledge and technology. Therefore it is extremely rewarding to maintain a high quality data transmission through the Internet, and it is also an enormous challenge for current engineers to focus research on a highly reliable communication system.
A schematic diagram for two user systems 10A and 10B to communicate through a transmission line 18 is shown in FIG. 1. Either one of the user systems could be a data switching system such as a circuit switching or a package switching system, a router, or a terminal. The transmission line 18 could be a network transmission line such as an Unshielded Twisted Pair (UTP) 5 of Ethernet. The user systems 10A and 10B comprise transformers 16A and 16B, and resistors R0a and R0b respectively to match the impedance of the transmission line 18. A transmitter 14A of the user system 10A generates a transmission signal with differential mode, which means the differential transmission signal comprises a positive and a negative transmission signal out of phase with each other. The positive and the negative transmission signals of the differential transmission signal are correlated to achieve a distant transmission through two connecting wires. The positive and the negative transmission signals of the differential transmission signal from transmitter 14A are output to node P0A and node P1A respectively and are coupled to the transmission line 18 by a transformer 16A. The differential transmission signals are then transmitted to the user system 10B by way of two connecting wires. Thereafter, the transmission signals are coupled to nodes P0B and P1B respectively by a transformer 16B and are received by the differential inputs of a receiver 14B. As a result, the user system 10A is able to transfer data to the other user system 10B through the transmission line 18.
However, there are some problems to be solved in the above described signal transmission process. For instance, although the transformers 16A and 16B are utilized to match the impedance of the transmission line 18, the feature of high pass filtering of transformers will diminish low frequency components of the transmission signals, which thus cause the voltage levels of the transmission signals to drift. Take an Ethernet network for example; some coding process such as MLT-3 coding must be done on the transmission signal for data transmission to the other user system before the signal is coupled to the transmission line by the transformer. After coding, there is a long-term average composed of composed of the low frequency component of the transmission signal. The amplitudes of low frequency components are related to the digital data with different levels in the transmission signal. When the transmission signal passes through the transformer 16A to the transmission line 18, the low frequency component of the transmission signal will be filtered by the transformer 16A which functions as a high pass filter. That is, the long-term average will be removed from the transmission signal. Thus, the baseline wander occurs at the other user system receiving the transmission signal.
Please refer to FIG. 2, which has a horizontal axis representing time. FIG. 2 shows a schematic diagram of level drifting of a receiving waveform due to the baseline wander of a transmission signal. While transmitting digital data from one user system 10A to the other user system 10B, the waveform of the transmission signal at node P0A is shown to be the waveform S0 in FIG. 2. Different digital data in the transmission signal is signified by three different kinds of pulses with different levels. For instance, during the duration Tp, there are first pulses with a plurality of high level periods, and the first pulses represent a plurality of digital data “1” in the transmission signal. During the duration Tn, there are second pulses with low level which represents digital data “−1” in the transmission signal. During the duration Tz, there are third pulses with zero level periods, i.e. level L0, which represents digital data “0” in the transmission signal. Therefore the digital data of the transmission signal is generated by coding the transmission signal waveform S0 utilizing three different types of pulses representing “1”, “0”, and “−1”. After transmitting to node P0B through the transmission line 18 and transformers 16A and 16B, the long-term average of waveform S0 is shown to be a waveform D in FIG. 2 with a reference zero level L0 shown as a horizontal dashed line. During the duration Ta, since the number of first pulses with the high level is far larger than the number of second pulses with the low level, the long-term average of waveform S0 is getting higher and the waveform D increases gradually with time. The increasing waveform can be expressed by a mathematic formula of (1−c·exp(t/T)) wherein c is a proportional constant, T is a time constant, and exp is exponential function. In another situation, during the duration Tb, since the number of first pulses with the high level is about the same as the number of second pulses with the low level, the long-term average of waveform S0 is getting lower and the waveform D decreases gradually with time, in contrast to the duration Ta with the higher longer-term average. The decreasing waveform can be expressed by a mathematic formula of exp(t/T). Similarly, during the duration Tc, since the number of first pulses with the high level is again far larger than the number of second pulses with the low level, the long-term average of waveform S0 is again getting higher and the waveform D increases gradually with time, in contrast to the duration Tb with the lower longer-term average. The increasing waveform can again be expressed by a mathematic formula of (1−c·exp(t/T)).
As described above, the low frequency components comprising the long-term average of the transmission signal is filtered out while the transmission signal passes through the transformer 16A in one user system 10A and the transformer 16B in the other user system 10B. Hence, the received waveform of the transmission signal at node P0B in the user system 10B is actually the same as the waveform S shown in FIG. 2. In other words, waveform S can be obtained by subtracting the waveform D from the waveform S0. Because of the filter effect, the waveform D composed of the low frequency component is removed from the waveform S0. For that reason, The levels of the pulses of waveform S drift as shown in FIG. 2. The received waveform S appears the waveform S0 carried by the waveform D. Therefore, the baseline wander occurs, and the digital data carried by the transmission signal cannot be retrieved correctly. Generally, the receiver 14B of the user system 12B retrieves the digital data from the series of pulses of waveform S based on criteria levels such as the levels of dashed line Lp and Ln shown in FIG. 2. Pulse levels higher than the voltage level Lp are represented by a digital “1” of a first pulse, and pulse levels lower than the voltage level Ln are represented by a digital “−1” of a second pulse. However, as the level wander of the pulses of waveform S occurs due to the baseline wander of the transmission signal at node P0B, the receiver 14B is not able to retrieve the digital data from the transmission signal correctly. For example, during the durations Te1 and Te2, due to the loss of low frequency components comprising the long-term average (i.e. waveform D), the downward offset of waveform S causes the levels of the first pulses to shift down below the voltage level Lp. For that reason, a wrong, interpretation of the first pulses, which are supposed to have higher pulse levels than voltage level Lp, occurs due to the baseline wander, and the data transmission is mistaken. Although the baseline wander phenomenon demonstrated in FIG. 2 is based on the positive transmission signal of the differential transmission signal transmitting from node P0A to node P0B, similar phenomenon happens to the negative transmission signal transmitting from node P1A to node P1B, as is well known to those skilled in the art.